Spectroscopy has long been utilized as a valuable investigative tool in various scientific fields. In particular, biological and medical research is conducted based on spectroscopic equipment which takes advantage of the underlying principles of selected wavelengths, such as the near infrared range. Examples of spectroscopic devices and applications thereof are disclosed and described in the patents and patent applications incorporated hereinabove by reference. Those skilled in the art will recognize other spectroscopic devices in applications in which the invention may be advantageously utilized.
It is highly desirable to employ LEDs in spectroscopic equipment. LEDs are relatively inexpensive to manufacture and are small in size. However, putting LEDs to use in the exacting environment of spectroscopic devices has presented significant difficulties which have heretofore not been sufficiently overcome.
Operating characteristics of diodes vary from unit to unit. For example, the wavelength of light emitted from different diodes are not identical. Such variations in wavelength are detrimental to the successful employment of LEDs in equipment for monitoring or testing biological substances. For example, spectroscopic devices used to analyze human bone, skin, or tissue, must have a light source emission within a known, narrow bandwidth to operate accurately. Accordingly, spectroscopic devices employing LEDs are carefully calibrated when the LED is installed.
One attempt at accounting for variations in emission frequencies of different LEDs used in spectroscopic devices uses an encoding component, such as a resistor of known resistance, selected to correspond to the measured wavelengths of light emitted from an associated LED. A detector which is used with the LED may identify the wavelength of light emitted by a particular LED by looking at its associated resistor.
Even if calibration and wavelength identification allow wavelength variations of diodes to be identified and compensated for at the calibration temperature, these techniques do not compensate for environmental changes, such as ambient temperature variations, of the LED. For example, in the novel oxymeter disclosed in U.S. patent application Ser. No. 07/711,147, an LED is placed in the proximity of a brain during surgery to monitor the oxygen level of the brain. In some operations, the brain is chilled during surgery. Because a thermal coupling exists between the sensor and the patient when the sensor is placed in the proximity of the chilled brain, the temperature of the diode junction will drop. With this drop in junction temperature, the intensity and wavelength of the LED emission will vary. This variation in wavelength and intensity causes substantial errors in the oxymeter measurements.
It is known to provide a resistive heating element to maintain a desired temperature under varying ambient conditions. However, for such a heating element to raise the temperature of an LED used in a spectroscopic sensor, the temperature externally of the sensor will also be raised. As a result, the temperature of the biological matter adjacent the sensor will also rise. Further, a substantial amount of circuitry and connectors would have to be used to implement such a heating element in a sensor for a spectroscope. Accordingly, it remains highly desirable to compensate for temperature variations occurring at the junction of an LED which is used in spectroscopic devices.